[Immunological functions of the intestines]

It was suggested just after the beginning of this century that the intestinal mucosa is equipped with its own immune system. However, it was not until 1965 that the major antibody molecule of exocrine secretions was characterized as a unique immunoglobulin called secretory IgA or SIgA. Our knowledge about local production and epithelial transport of SIgA has increased rapidly during the last 10-15 years. A fascinating co-operation between lymphoid B cells and glandular epithelial cells has been delineated. SIgA is in quantitative terms the most important antibody molecule and the intestine is the largest immunological organ of the human body. Antigen-stimulated B and T lymphocytes are distributed from Peyer's patches and other gut-associated lymphoid tissue to exocrine glandular sites all over the body; this is the basis for local generation of SIgA antibodies with an enormous selection of specificities required for protection of the extensive mucosal surfaces. Regulation of this integrated mucosal immune system is quite critical; break in the immunological control of mucosal homeostasis leads to local overactivation of T cells and overproduction of phlogistic IgG (and IgE in atopic individuals). The great immunological activity of the intestinal mucosa may thus in an imbalanced way contribute to tissue damage and inflammatory disease.